Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual, Page: 95 / 413

WSM.124
Section C (Fuel System)
I
st re-issue
Page 9
CARBURETTOR
GENERAL DESCRIPTION
The following carburettors and air cleaners are
fitted as standard equipment
Alpine
Twin Zenith WIP 36 downdraught carburettors
fitted with separate removable gauze type air
intake filters.
Rapier
Twin Zenith WIA 36 downdraught carburettors
connected to a single oil bath air cleaner by a
common air box.
WIP carburettors have the by-pass fuel supply
system blanked off. WIA carburettors liave a by­
pass jet supplied through a by-pass valve which is
controlled by a diaphragm and spring operated by
inlet manifold depression.
36 denotes that the throttle bore size is 36 mm.
and WI the carburettor type.
Both these carburettors have a modified accelerator
pump operating mechanism which gives a more
rapid action of the accelerator pump during early
stages of throttle opening.
Jet
sizes, orifice hole sizes and setting dimensions are
given in the Data Section at the front of this manual.
By removing six cheese-headed screws, the top body
of the carburettor can be removed, allowing easy
access to the following Items: float and float needle
and seat, slow running jet, the combined by-pas-:.
valve and jet, the accelerator pump and Its check
valve, discharge nozzle, and delivery ball valve.
The main jet can be removed by using a spanner on
its external hexagon head which Is situated on the
underside of the float chamber.
The three main components of the carburettor, the
various passage-ways, jets, valves and mechanical
linkage are shown in Figs. 5 and 6, An exploded
view
of the carburettor is given in Fig. 18 and the
names of the various parts on the adjoining page.
A heat insulating joint of 5 mm. thickness is used
between the throttle spindle body and the main
body, to reduce heat flow from the manifold to
float chamber and jets. This arrangement has the
advantage of keeping the throttle warm which
prevents icing up in cold damp weather. Four
¼
n B.S.F. recessed cheese-headed screws are used
to hold these parts together.
Under extremely hot operating conditions fuel will
sometimes boil in the carburettor after the engine
has been stopped. To deal with this the main
discharge jet drillings and high speed bleed are
arranged to act as an anti-percolating device keep­
ing the boiling fuel bubbles inside the carburettor.
CARBURETTOR OPERATION
The carburettor has to supply a correct fuel/air
mixture to meet the following operating conditions.
1. Good starting from cold.
2. Slow running or fast Idling.
3. Cruising (part throttle running).
4. Acceleration without flat spots.
5. Full power (full throttle) requirements.
The way in which these requirements are met by
the various parts of the carburettor are fully
described and illustrated in the following pages.
Float Chamber (See Fig. 7)
Petrol enters the carburettor through the Inlet at
the base of the float chamber, and then passes
through the float needle seating. The needle is
attached to the cylindrical float by means of a
wire clip. As petrol ls admitted, the float rises and
pushes the needle down to Its seat, thus cutting off
the flow when the correct level is reached In the
float chamber. This action is entirely automatic and
continues all the time the carburettor is being fed
with fuel.
From the float chamber fuel passes to the main jet,
accelerator pump suction valve, and to the by-pass